Several varieties of kilns are known in the prior art for the calcination of limestone and dolomite, to manufacture lime or magnesite, respectively, said kilns carrying out the calcining process by many different procedural steps, and vary from mere cupola furnaces wherein the limestone or the dolomite are calcined by means of the countercurrent passage, with respect to a granulated material column, of combustion gases at a temperature sufficient to accomplish such a calcination, up to highly efficient kilns producing a first quality and very uniform material, as well as capable of recovering most of the waste heat which escapes with the exhaust gases in the calcining process.
The primitive cupola furnaces, however, have left must to desire because the efficiency of the process is so low that, given the low price of the materials normally calcined by means of said furnaces, which renders it impossible to permit increases in the costs and low efficiency in the kiln for carrying out said processes, they have not been capable of permitting an economical operation and, consequently, such primitive furnaces or kilns have been practically disregarded and have been displaced from the market by more efficient kilns.
The known kilns for carrying out strongly endothermic processes, such as melting furnaces, kilns for the degasification of carbonates, kilns for the calcination of limestone and dolomite, and the like, such as is well known in the prior art, have suffered of a very low thermal efficiency, due to the large waste of heat in the exhaust gases. Said losses have been higher, the higher the excess amounts of air used are in the kilns of this type, and the thermal efficiency is very low regardless of the type of fuel utilized, either liquid, solid or gaseous.
The reasons for the thermal losses in the prior art kilns to carry out endothermic processes of this type, resides on the waste of burned gases which are exhausted from the reaction zone, in which the actual combustion is effected at very high temperatures, thereby carrying out enormous amounts of heat that the material in the preheating zone does not absorb in a sufficient manner, due to the fact that the temperature differentials are not sufficiently high to transfer reasonable amounts of heat from said combustion gases to the material.
This is particularly serious when dealing with heat sensitive material such as limestone or dolomite, wherein the necessarily low temperatures for the reaction can be maintained only with great excess of air or by means of the addition of large amounts of inert gases, for instance, kiln exhaust gases, which still increases to a larger extent the amounts of heat which are carried by the gases leaving the reaction zone, thereby additionally reducing the efficiency of combustion in the process, with the resulting high temperatures of the exhaust gases.
In order to overcome such large heat losses, processes have been suggested wherein, between a reaction zone and a material preheating zone, waste gases are extracted and reintroduced in the reaction zone in a suitable point, either alone or in the presence of cold air. While in this manner the heat losses are considerably reduced, there is the inconveniency that the bypass for these gases normally tends to loose heat through radiation, whereby a very good installation is required, thus increasing the costs of production, and also this method cannot reduce the losses in the waste gases resulting from the use of the necessary excess of air in the combustion of the fuel.
In order to overcome these disadvantages a double column kiln was designed to carry out the endothermic processes of the above described nature, said kiln being suitable for instance for the production of lime and said process being carried out in a double column kiln in which the material to be calcined is preheated in a first column by the countercurrent passage of combustion gases leaving the reaction zone of the second column in an intermediate position between said columns, whereas in the second column fuel and primary combustion air are introduced, which primary air is preheated by the previously preheated material in said column, and burns the fuel in order to effect the calcination, at the same time injecting cold air at the lower end of said second column in order to pass said cold secondary air, jointly with the combustion gases, to the first column to preheat in a countercurrent contact the material which descends through the same. After a predetermined period which is generally of from 10 to 20 minutes, the flow to the columns is reversed and then the first column acts as a calcining column whereas the second column acts as a preheating column. As at the lower portions of said column only cold air is circulated, both columns in this zone constitute cooling zones for the material which is being calcined, which material is discharged after cooling from the bottom of said column, although in an alternated manner, that is, first from one column and thereafter from the other column.
Although the thermal efficiency of this type of kilns is high and the quality of the thus obtained material is uniform and also high, said kiln present the serious drawback that the operation of the kiln is extremely intricate and requires flow reversing installations, as well as due attention from the generators or automatic controls which may be susceptible to damage, inasmuch as the process must be reversed within very accurate and predetermined periods of time, with the consequent cumbersome actions and, if a careless operation is effected, the devices built in accordance with this technique very probably will malfunction.
The great advantage contributed by the prior art double column kiln and process described above, which is particularly described and claimed in U.S. Pat. No. 3,074,706 patented Jan. 22, 1963 to Schmid et al, is that for a first time a zone for preheating the material is introduced, which is absolutely separated from the reaction zone, whereby an effective preheating of the material is accomplished, as well as a good reaction of the previously preheated material, which increase the efficiency of the reaction. However, the separation of these two zones, namely, the preheating and the reaction zone, by the use of two parallel and separate columns, presents serious drawbacks as to its operation and complicates the process in a manner such that the production costs are considerably increased.
Therefore for long the workers in this art have sought to design a kiln which, without the need of having to reverse the flow for alternating preheating and reacting in the two separate zones described above, may accomplish the separation of the preheating zone with respect to the reaction zone in an efficient manner, but without presenting the drawbacks of the prior art kilns and processes. Up to the present date, however, said task has not been accomplished, because the only method and the only kiln existing up to the present date to carry out the preheating and the reaction in totally separated zones, is the one described and claimed in the above mentioned U.S. Pat. No. 3,074,706 to A. Schmid et al, which constitutes up to the present time the most popular and commercially used kiln, as may be proven by the kilns that are being installed in most of the world by the Kennedy Van Saun Corporation, with its kiln MCV (Multi Column Vertical) which is denominated multiple column vertical kiln, and which is a two column, parallel flow regenerative kiln such as described above.